The present invention is generally directed to BALUN transformer circuits for analog signals. More particularly, the present invention is directed to the construction of BALUN circuits which are fabricated on the integrated circuit substrate. Even more particularly, the present invention is directed to BALUN transformer structures which are disposed in a plurality of layers so as to make the BALUN structures which are not only compact but which also reduce parasitic capacitance effects between primary and secondary transformer circuits.
A BALUN is an electronic circuit component which is employed to provide impedance balance in radio frequency (RF) circuits. In essence, the BALUN is a balanced to un-balanced transformer. These devices are used in radio frequency applications to connect un-balanced components and circuits to balanced ones.
Unbalanced circuit components are usually large and expensive and include such devices as filters and power amplifiers. To make them balanced, while possible, would require them to be twice as large and expensive. On the other hand, balanced components are usually found in integrated circuits because the cost of doubling the number of internal components is low in an integrated circuit structure. BALUN circuits also have desirable electrical properties. They have improved signal isolation and linearity characteristics and, accordingly, are used whenever possible. In an un-balanced circuit, the current is typically carried by a single wire and returned through a ground which is preferably a large conductor which either forms the chassis for the device or is otherwise electrically connected directly to the chassis. On the other hand, a balanced circuit typically includes two conductors and a ground. In the BALUN case, current flows xe2x80x9cdownxe2x80x9d one conductor and returns via the other one. This eliminates current flowing through the ground. This is the ideal situation.
BALUN circuits are desired in radio design because chip designs tend to be differential while external components tend to be single ended. Hence, a BALUN transformer is needed to couple internal and external components. However, BALUN""s generally have limitations with respect to cost and size particularly when it comes to their use in radio frequency devices such as cellular telephones. These devices clearly require BALUNS that exhibit small sizes particularly with respect to the other components in a cellular telephone or cellular telephone system. Hence, there is a need for small inexpensive BALUN transformers that also exhibit good electrical performance. In particular, it is highly desirable that integrated radio frequency integrated circuits employ differential circuits which tend to improve isolation to common mode signals. Furthermore, power matching requires the use of balanced impedance transformers. Additionally, passive as opposed to active BALUN devices are desirable because they do not require additional current for their operation. However, passive baluns tend to be physically very large. The present BALUN invention allows for size reduction in passive BALUN devices.
In accordance with a preferred embodiment of the present invention, a radio frequency BALUN transformer comprises a substantially flat primary conductive loop and a substantially flat secondary conductive loop. These conductive loops are each configured in spiral patterns. The loops are disposed so that at least a portion of one of the loops is positioned so as to be horizontally adjacent to a portion of the other loop. Additionally, at least one of the loops is disposed in at least two vertically adjacent planes. The loop portions in vertically adjacent planes have the same winding orientation. In one particular embodiment of the present invention, both of the loops have portions which are disposed in vertically adjacent planes. The BALUN transformers of the present invention are particularly useful in high frequency applications; for example, a BALUN circuit useful at a frequency of approximately 850 megahertz can be constructed in a rectangular pattern which is only 235 microns on each side.
Accordingly, it is an object of the present invention to provide compact BALUN transformer circuits.
It is also another object of the present invention to provide impedance matching in high frequency circuit applications without the utilization of active circuit components.
It is yet another object of the present invention to reduce the parasitic capacitance that exists between the primary and secondary windings of a BALUN transformer.
It is a still further object of the present invention to provide BALUN transformer circuits which are particularly useful in cellular telephones and cellular telephone systems.
It is also an object of the present invention to provide BALUN transformer circuits which may be fabricated using integrated circuit fabrication procedures.
It is also an object of the present invention to provide a BALUN circuit which may be fabricated in two or more planar circuit structures.
It is yet another object of the present invention to provide a BALUN circuit which is small, effective, easy to manufacture and which is of low cost.
Lastly, but not limited hereto, it is an object of the present invention to provide BALUN circuits having a variety of impedance characteristics.
The recitation herein of a list of desirable objects which are met by various embodiments of the present invention is not meant to imply or suggest that any or all of these objects are present as essential or necessary features, either individually or collectively, in the most general embodiment of the present invention or in any of its more specific embodiments.